Condenser microphone



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INVENTOR. 2

ERW/N G LA CH MW ATTORNEY;

Patented Sept. 29, 1942 2,297,211 CONDENSER MICROPHONE Erwin Gerlach, Berlin, Germany; vested in the Alien Property Custodian Application July 15, 1939, Serial No. 284,647 In Germany July 25, 19 38 1' Claim.

In condenser or electrostatic microphones of high sensitiveness the space intervening between the surfaces of .,the electrodes'amounts to only a few microns. Hence, there is a risk of this gap undergoing alterations asa result of unequal thermal expansion ofthe electrode supports.

This risk is eliminated according to the invention by making the said electrode supporting or holding means of the same material or of a kind of material which has the same heat expansion coefficient, and by mounting them upon a common plate or slab of insulating material. Hence, figured from the surface of the said plate as the reference surface, there will thus' arise 'no differences in thermal expansion in the direction towards the face of the electrodes.

The invention shall now be described in more detail by reference to an exemplified embodimerit thereof illustrated in the appended drawing. Both figures of the drawing show sectional views of condenser microphones.

Referring to Fig. 1, one of the electrodes I consists of a diaphragm which by means of a screwthreaded ring 2 and interposition of an annular washer 4 is screwed fast upon the electrode support or mount 3. The second electrode 6 is of the apertured or perforated (sieve) type. It is secured to the electrode support Ill. The electrode support 3 has an offset I3 which comes to bear upon-the insulation plate 5, while electrode support I bears with its underside II upon the same insulation plate 5. The latter is in pressure engagement with the offset or recess I3 by means of the screw-threaded ring I5, with an annular washer I4 being interposed. Connection between the electrode support III and the insulation plate 5 is established by means of a screw 9.

The electrode supports 3 and III are made of the same material, that is, brass. The insulation plate 5 consists of glass or some other insulator such as amber. The upper face of the insulation plate 5 which bears against the surfaces II and I3 is polishedso as to be perfectly smooth and level.

If, then, upon fluctuations of the temperature, the electrode mount I0 experiences expansion,

the electrode support 3 will expand similarly,

and the result is that the distance or gap between the electrodes I and 6 will always be constant and stable.

Fig. 2 shows a similar embodiment in which, however, the first or outer electrode support consists of several constituent parts rather than only one part. The diaphragm I is shrunk-fitted by means of a ring 2 and another, interposed ring 4 upon the supporting ring 3, the latter, in turn, being united with the support 30 by means of the screw 2-3. The offset portion 3| of the support bears upon the surface of the insulation plate 5. The second perforated electrode 6 is secured upon the electrode support or holder part III which rests upon a yielding plate I6, which, on the other hand, comes to bear upon a projection Il of the insulation plate 5. In this embodiment the distance separating the electrodes I and 6 is adjustable by the aid of a set screw 9. Parts 3, 30, I0 and I6 consist of a sort of material that has the same coeflicient of thermal expansion.

What is claimed is:

A microphone comprising a diaphragm forming a movable electrode, a shouldered support for said movable electrode, a second electrode, a support for said second electrode located within said shouldered support, an insulating plate for supporting said second electrode and located at said shouldered portion of said first mentioned support, projections on said insulating plate, a resilient plate resting on said projections, and means associated therewith to adjust the spacing between the electrodes, both of said supports being of material having the same thermal expans'ion coeflicient whereby there will be substantially no change in the spacing between said electrodes due to thermal expansion of said supports.

ERwIN curamcrr. 

